Normally specifying a _blank in actionscript will result in opening a new window, so if it is not doing that, it's not likely you will be able to fix it if it is an internal design issue of some Dreamweaver feature.

Well, that's not very good. If it's a Dreamweaver feature, then one of the Dreamweaver experts should be able to manage answering (you may have to wait for one of them to come along). Some of the folks in that forum are more web design experts than DW masters. Folks In the Flash forums are more likely to build image viewers of their own than to use one from DW, but you never know... you may get lucky here too. I am not familiar with it, so the best I could offer has already been.